Background: The purpose of this study was to analyze cardiac output (Qc), stroke volume (SV), heart rate (HR), and arterio-venous O<inf>2</inf> difference (a-vO<inf>2diff</inf>) responses throughout a graded exercise test (GXT) and verification phase (VP) to examine whether SV decrement during the GXT is a main factor for underestimation of the maximal O<inf>2</inf> uptake (V̇O<inf>2max</inf>), or not.
Methods: Seven well-trained male cyclists volunteered for this study (V̇O<inf>2max</inf>: 61.7±6.13 mL∙min-1∙kg-1). Following submaximal tests, participants were asked to perform GXT until exhaustion. Then, multisession verifications were performed on different days using ±3% constant work rates. The highest 30-second mean of V̇O<inf>2</inf> was considered as the V̇O<inf>2max</inf> and corresponding external power as peak power output (PPO). The Qc, SV, HR, and a-vO<inf>2diff</inf> responses were evaluated at both GXT and VP by nitrous-oxide rebreathing method. After repeated-measures analyses, possible significant differences were investigated by LSD/Wilcoxon.
Results: It was shown that the HR and a-vO<inf>2diff</inf> reached their potentially highest values at the end of the both GXT and VP (192.9±8.8 vs. 190.7±7.9 bpm; 17.1±1.6 vs. 16.9±1.1%, respectively; P>0.05); however, SV (128.8±11.2 vs. 137.3±11.2 mL; P=0.029) and Qc (24.8±2.02 vs. 26.2±2.71 L·min-1; P=0.046) were lower at GXT when compared to the VP. V̇O<inf>2</inf> means were, therefore, higher in VP when compared to the GXT (61.7±6.13 vs. 59.1±6.2 mL∙min-1∙kg-1; P=0.041).
Conclusions: The GXT provided only a peak V̇O<inf>2</inf> but not the V̇O<inf>2max</inf>. Consequently, the real V̇O<inf>2max</inf> and PPO could be provided by only VP administrations. This is likely to result from the lower Qc and SV responses observed from a prolonged incremental test protocol when compared to short bouts of constant work rate trials.